Analog-to-digital (A/D) converters are in widespread use today in electronics for consumers, industrial applications, weapons, etc. Typically, A/D converters include circuitry for receiving an analog input signal and outputting a digital value proportional to the analog input signal. This digital output value is typically in the form of either a parallel word or a serial digital bit string. There are many types of A/D conversion schemes such as voltage-to-frequency conversion, charge redistribution, delta modulation, as well as others. Typically, each of these conversion selected has its advantages and disadvantages.
One type of A/D converter that has seen increasing use is that utilizing delta-sigma modulation where an analog voltage is input to a delta-sigma modulator and the output thereof filtered to remove noise. A delta-sigma modulator typically converts an analog input to a digital pulse string having an average amplitude over time proportional to the analog input. Delta-sigma modulation generally provides for high accuracy and wide dynamic range as compared to earlier delta modulation techniques. Delta-sigma modulation is often referred to as an oversampled converter architecture and is typically immune from some of the earlier undesirable second order effects of delta modulation.
There are generally two key components of a delta-sigma A/D converter, an analog modulator and a digital filter. The analog modulator oversamples the analog input and produces a digital output. However, with generally any A/D converter there are a number of noise sources that are inherent to the design. In a typical delta-sigma modulator, there are output stage noise sources and input stage noise sources, the output noise sources normally being dominated by quantization noise and the input noise sources resulting from 1/f noise. In addition, there may also be present thermal noise and a deterministic error from a DC offset that is a static error. The quantization noise at low frequencies is relatively low with the largest portion thereof existing at higher frequencies. This higher frequency portion noise can be filtered out by a digital domain low-pass filter. However, low frequency DC offset and 1/f noise cannot be filtered out by a low-pass filter, and, as such, such effects are typically passed through the filter with the signal information.